This invention relates to an improved process for the preparation of a modified aromatic hydrocarbon resin, by reacting an aromatic hydrocarbon-formaldehyde resin with an unsaturated polybasic acid or an anhydride thereof, if desired, together with a saturated polybasic acid or an anhydride thereof. More particularly, the invention relates to a process characterized in that water or water and an aliphatic alcohol is made present in the reaction system during the reaction and the reaction is made to proceed while removing the formaldehyde formed by the reaction.
It has hitherto been widely known that a modified aromatic hydrocarbon resin, which may be hereinafter referred to briefly as "modified resin", can be produced by reacting an aromatic hydrocarbon-formaldehyde resin with an unsaturated polybasic acid or an anhydride thereof such as maleic acid or maleic anhydride. This modified resin may be also regarded as a modified unsaturated polyester, which may be dissolved in monomers such as styrene to be used for purposes similar to those of conventional unsaturated polyester resin.
In case, however, an aromatic hydrocarbon-formaldehyde resin, e.g. xylene-formaldehyde resin, is reacted with an unsaturated dibasic acid anhydride, e.g. maleic anhydride, to prepare said modified resin, the difficulty lies in the fact that gelation tends to occur during the reaction. It was proposed to use a specified amount of the unsaturated dibasic acid or an anhydride thereof in order to overcome the difficulty (see Japanese Patent Publication No. 33,786/71). Although the gelation can be fairly prevented by the proposed method, the cured product has a low crosslinking density and insufficient mechanical strength. Therefore, the modified resin as above-mentioned is not satisfactory for use as a resin for fibre reinforced plastics (FRP) in particular.
In order to obtain a modified resin suitable to the use as the resin for FRP in particular, we have studied processes for its preparation carefully and found that the gelling tendency in the final period of the reaction and the low reactivity of the modified resin obtained according to the proposed method are due to the formaldehyde formed in the course of reaction.
It might be sufficient if the formaldehyde formed by the reaction is not made present in the reaction system, but it is undesirable to discharge the gaseous formaldehyde into the atmosphere, since it contaminates the working environment. Furthermore, in case a discharge pipe, or the like is employed, such measures would also be unfavorable since the formaldehyde deposits on the inside wall of the pipe and clogs the pipe. Thus, any of the cases is not satisfactory for the commercial practice.